1. Field of the Invention
This invention pertains to the field of navigation and more specifically to a navigation system employing only inertial components.
2. Description of the Prior Art
Prior art passive navigation systems utilizing only gyroscopes and accelerometers do not provide the continuous velocity, position, and attitude accuracy generally required for the long term operation of such systems. Position, velocity, and attitude errors caused by drift and gravitational effects on these inertial sensors, rendered them unacceptable as a sole sensors in a navigation system for operation over the long term. Operation over the long term of these prior art inertial navigation systems required periodic updates of position. These updates were generally provided by fixes from the Global Positioning System (GPS), a radar navigation system, or a sonar system.
Significant improvements have been made in inertial instruments such as gyroscopes and accelerometers. Drift has been reduced to insignificant levels leaving only the gravitational effects as the major source of error. Though gravimetric maps are available for the correction of inertial sensor performance, highly accurate corrections can be made with the use of these maps only if the position of the vehicle is precisely known and the maps are error free. Additionally, the vertical gravitational field which is deflected by the coriolis effect is further deflected by the motion of the vehicle. Vertical deflection, create horizontal components, which are known as horizontal gravity anomalies. These anomalies impact on inertial navigation systems very much like accelerometer errors. As the vehicle traverses through the anomalous gravity field, the Schuler loop is excited and velocity and position errors are generated which increase with time. Consequently, if a completely inertial navigation system is to provide sufficient accuracy over the long term, inertial sensor errors, caused by anomalous gravitational fields, must be corrected in real time.
Processing gravity anomalies in a Kalman filter requires complicated modeling. Consequently, inertial navigation systems of the prior art operate in conjunction with electromagnetic systems or external navigation systems, such as the Global Positioning System (GPS) to receive periodic updates from these systems. An inertial navigation system of the prior art, disclosed in U.S. Pat. No. 5,272,639 issued to J. T. McGuffin on Dec. 23, 1993, periodically utilizes three sets of geo-physical correlation data to provide updates to a primary inertial navigator. A Kalman filter is used to correlate terrain data, electromagnetic data, and gravimetric data with the output of a digital map of terrain, electromagnetic field, and gravity. A best of three selection process, based on the errors in the Kalman filter, decides whether to accept the prediction of the terrain system, electromagnetic system, or the gravity system to update the inertial navigator.